The goal of this study is to achieve a deeper understanding of the mechanisms involved in differential parent/child interactions. Differential parental treatment may have important effects on siblings in terms of cognitive development, psychopathology, and sibling relationships. In particular, is it parental personality or child temperament which is more important in driving the interactions? A novel approach involves the study of twins, allowing an analysis of differential parent behaviors toward two same-age children at the same, point in time. In addition, the use of twins permits a test of whether parents treat identical twins more similarly than fraternal twins and whether this continues over time, thus allowing for a test of the importance of child behavior on the parent/child interaction. Parent/child interactions will be assessed longitudinally when the children are 5, 7, 9, 20, 24, and 36 months old. Ratings will be based on videotaped interactions in which parents are told to elicit vocalizations from the infants (at 5 to 9 months) or in which parents are involved in a teaching task attempting to instruct their two children simultaneously on how to place shapes on a pegboard (at 20 months) or how to sort objects by color (at 24 months) or by shape and color (at 36 months). Parental ratings will assess parent's warmth, verbal responsiveness, and control using an established coding scheme, as well as verbal behaviors comparing one child to the other and parental responses to children's bids for attention using a scheme which will be developed as part of this project. The role of parental personality and child temperament as they affect differential parent/child interactions will be of primary importance to this study. The performance of the children (number of vocalizations produced, number of shapes placed on pegs, or number of objects correctly sorted) during the interactions will provide an outcome measure to determine the effectiveness of parents' instructions. Parental and child influences on parent/child interactions and the subsequent effects on child behaviors will be assessed. Thus, the effects of personality and temperament on parent/child interactions may impact indirectly on the well-being of the child by affecting the child's, behaviors (outcomes) as a result of the interactions.